Monday 08 Dec 2014Recent advances in sub-seasonal predictions

Gilbert Brunet - Met Office/ Environment Canada

Harrison 103 15:00-16:00

The sub-seasonal to seasonal variability at mid-latitude that is characterized by weather regimes (e.g. Atlantic blockings) and large scale oscillations and patterns, like the North Atlantic Oscillation (NAO) and Pacific/North American (PNA). These weather regimes modulate regionally the occurrence of high-impact weather like the winter 2013-14 extreme cold event over North America and exceptional spells of winter storms associated with severe coastal damage and sustained flooding in the United Kingdom. Positive NAO phases are associated with stormy weather in the UK. It is believed that these events were associated with the exceptional rainfall observed over the West Pacific, Indonesia and the eastern Indian Ocean during December and January 2013-14 (Slingo et al., 2014). Similar tropical rainfall patterns have been correlated significantly with positive phase of the NAO in observations and model output by Lin et al. (GRL, 2005). The latter study identifies an increasing trend in this relationship for the 51 winters of 1948/49 to 1998/99. The Quasi-Biennial Oscillation (QBO) is an inter-annual tropical mode of variability that also can increase the seasonal occurrence of positive NAO phase and strength of the jet stream (Slingo et al., 2014).

The Madden-Julian Oscillation (MJO) is the dominant mode of sub-seasonal variability in the tropics, which has a direct impact on the weather in the tropical region, as it organizes convection and precipitation. It also has a significant influence on the extratropical atmospheric variability through Rossby wave propagation and thus provides an important signal source for mid-latitude weather forecasts on sub-seasonal time scales. In the work of Lin et al. [2009], it was found that there is a significant two-way interaction between the MJO and the NAO. By analyzing the output of sub-seasonal hindcasts, Lin et al. [2010] and Lin and Brunet [2011], it was demonstrated that this two-way interaction was important for Numerical Weather Prediction (NWP) forecast skill of the MJO and NAO.

We will discuss recent research progress to clarify the role of these different tropical and mid-latitude modes of variability in terms of predictability and dynamical processes, weather impacts and predictive skill for the sub-seasonal to seasonal prediction problem.

1. Slingo et al., 2014: The recent storms and floods in the UK, Met Office and Centre for Ecology and Hydrology internal report, 1-27.
2. Lin, H., and G. Brunet, 2011: Impact of the North Atlantic Oscillation on the forecast skill of the Madden-Julian Oscillation. Geophys. Res. Lett., 38, L02802, doi:10.1029/2010GL046131.
3. Lin, H., G. Brunet and J. S. Fontecilla 2010: Impact of the Madden-Julian Oscillation on the intraseasonal forecast skill of the North Atlantic Oscillation. Geophys. Res. Lett., 37, L19803.
4. Lin, H., G. Brunet, and J. Derome, 2009: An observed connection between the North Atlantic Oscillation and the Madden-Julian Oscillation. J. Climate, 22, 364-380.
Lin, H. J. Derome and G. Brunet 2005 Tropical Pacific link to the two dominant patterns of atmospheric variability. Geophys. Res. Lett., 32, L03801.

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